(1) Field of the Invention
The present invention relates to telephone trunk circuits and more particularly to a detector circuit for use in a digital private automatic branch exchange, to detect supervisory signals supplied by a central office. The present circuit designed for inclusion in a PABX trunk circuit, provides maintenance indications concerning the integrity of the leads which connect the central office to the PABX.
(2) Description of the Prior Art
In a telephone system it is necessary that a central office provide supervisory signals via the trunk circuit to the PABX. For example, these signals indicate to the PABX that it is to transmit the calling number or the called number. This signalling is accomplished via battery polarity reversals applied at the central office to the tip and ring leads of the trunk circuit.
Existing trunk circuits typically sense such polarity changes by the use of a polar relay which senses only a single polarity change. Such polar relays are expensive, consume relatively large amounts of space and are generally of lower impedance than desirable. Because of the combination of these factors, only limited status monitoring features are provided in most existing trunks. Furthermore, the use of this type of relay does not provide for sensing subsequent polarity reversals in both high impedance and low impedance modes, required for trunk circuit operations.
A partial solution to these problems is taught by U.S. Pat. No. 4,002,850 issued to G. L. Richards et al, which demonstrates the use of an optical-coupler for the detection of single battery polarity reversals initiated at the central office. No positive indication of normal battery detection is taught.
Therefore, it is the object of the present invention to provide a trunk circuit detector for detecting both normal and reverse battery polarity for both high impedance (idle) and low impedance (busy) modes of operation of the trunk circuit, for the detection of fault conditions such as, an electrically shorted tip and ring lead, an open circuit of tip and ring leads, an electrical ground placed onto the ring lead and for a bidirectional loop current threshold sensor.